In electrophotographic printers and copiers, a toner image is formed on the surface of a photoconductive recording element. This image is commonly transferred, either directly or indirectly, to a receiver member (e.g., a sheet of paper), thereby enabling the recording element to be re-cycled through the image-forming process to make multiple prints/copies. Often, the physical form of the recording element is that of a drum having an outer surface of photoconductive material, either organic or inorganic. As the drum rotates, various work stations positioned about the drum periphery operate collectively to produce the toner image on the drum's photoconductive surface. These work stations usually comprise (i) a primary charging station for depositing a substantially uniform electrostatic charge on the drum's photoconductive surface; (ii) an exposure station for imagewise exposing the uniform charge to actinic radiation, thereby selectively dissipating the uniform charge to produce a charge image; and (iii) a development station for applying pigmented thermoplastic particles (toner) to the charge image to render it visible. In addition to these image-processing stations, other work stations, also positioned about the drum periphery, serve to transfer the toner image thus formed to an image-receiving member, e.g., a sheet of paper, or to an intermediate transfer drum from which it may be subsequently transferred to paper or the like, and to remove residual or non-transferred toner from the drum's photoconductive surface prior to recycling the drum through the image-forming process. When using an intermediate transfer drum, an additional toner-cleaning station is positioned adjacent the intermediate transfer drum, downstream from the second image-transfer station, to remove residual toner particles.
As will be appreciated, the consistent production of high quality images requires that certain positional relationships be established and maintained at all times between the above work stations and the photoconductive drum and intermediate transfer drum. For example, the spacing between the drum surface and the corona discharge wire(s) of the primary charging station must be maintained uniform across the drum surface in order to assure a uniform charge distribution across the drum surface. Further, the spacing between the drum surface and a toner-applying magnetic brush (or the like) must be kept within a very tight tolerance to consistently achieve a desired toner image density. The same holds true for spacing between the drum surface and the exposure station, which may be in the form of a solid-state print head or an optical projection system, in order to consistently form a sharply focused image on the drum's photosensitive surface. In some printer/copiers, such positioning of the work stations relative to the recording drum is maintained by using wheels that contact and rotate on the drum's outer surface. In other machines, reference rings or other structures are used to maintain the desired spacing. All such approaches require the use of precision parts, and are problematic from the standpoints of contamination and wear.
One example of positioning apparatus of the above type is disclosed in U.S. Pat. No. 5,089,846 to H. Tabuchi. In this disclosure, an exposure station in the form of an array of light-emitting diodes is supported for pivotal movement towards and away from a photoconductive drum. The positioning mechanism is mounted on the pivotal support and has an end that is adapted to contact and ride upon the outer edge of the photoconductive drum to thereby establish a desired spacing between the drum surface and the operative surfaces of the LED array. A second support pivotally mounted on the first support is spring biased to urge the first support towards engagement with the drum surface, and a cam surface mounted on the machine frame interacts with the second member to adjust its pivotal position. While intended to provide a simple and inexpensive approach to achieving high positional accuracy between the drum surface and the operative surface of the LED array, this approach is still subject to many of the aforementioned disadvantages, requiring the use of precision parts that eventually wear-out and introduce contamination.
An example of an improved positioning apparatus that is not subject to the aforementioned disadvantages is disclosed in U.S. Pat. No. 6,427,059 to Buch, et. al. Such apparatus includes a pair of drum-support members, each having associated reference surface features adapted to mate with complimentary reference surface features on the individual work stations. The placement of the reference surface features of the drum-support members are factory set to account for any idiosyncrasies (e.g., run-out) of a drum supported by such members. This apparatus does not, however, provide for adjustment of the position of the various work stations relative to photoconductor element at the user site if such adjustment should be determined to be required, for example, by observation of image quality deficiencies and/or the result of prescribed diagnostic procedures.